drm/amdkfd: Check if there are kfd porcesses using adev by kfd_processes_count

drm/amdkfd: Check if there are kfd porcesses using adev by kfd_processes_count

During gpu hot-unplug need check if there are kfd porcesses still using the
being removed gpu before clean resources of the device. Current driver checks
if kfd_processes_table is empty. kfd processes are not terminated after
removed from kfd_processes_table immediately. They are still alive and may
access the device until kfd_process_wq work queue got ran.

Check kfd->kfd_processes_count value that is updated after kfd process got
uninitialized when its ref becomes zero.

Fixes: 6cca686dfce7 ("drm/amdkfd: kfd driver supports hot unplug/replug amdgpu devices")
Signed-off-by: Xiaogang Chen <xiaogang.chen@amd.com>
Reviewed-by: Felix Kuehling <felix.kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>